Most of the familiar animals (bears, people, fish, insects, worms, squid, snails, starfish) are members of the bilateria, bilaterally symmetric animals with front and back, top and bottom. Most of them are either deuterostomes (e.g., us, fish, starfish) or protostomes (arthropods, worms, clams, squid).
They all develop from an egg which divides and forms a hollow ball of cells which dents in during gastrulation and ultimately penetrates to the other side, forming the gut. In deuterostomes, the original dent becomes the anus. In protostomes, it either becomes the mouth (or according to some) both mouth and anus.
One of us seems to have got it backwards. How did that happen?